1
cbi
C
bb
R
be
C
'
'
e
b
m
V
g
be
R
ex
R
+

'
'
e
b
V
cbx
C
ECE194J/594J PS 3.
f
m
depl
be
be
g
C
C
τ
+
=
,
Above is a device
model . Recall that
.
Let us take Cje=5.67 fF, beta=50,
Ccbi=1.86 fF, Ccbx=1.34 fF, Rbb=14.7
Ohms, Rex=8 Ohms, and tau_f=0.244 ps.
Lets use this device model in circuit
calculations below.
nKT
qI
g
m
/
=
,
where n=1.0. This is the model of a
transistor having Ae=0.25 um x 4.0 um
emitter area and biased at 1 mA/micron
current density (4 mA total); the ft is 470
GHz ft and the fmax 825 GHz. It operates
at a maximum of 2.5
mA/um at Vce=1.0
volts.
In the hand analysis ONLY (for problems
1 and 2), we will simplify by making
Ccbi=(1.86+1.34) fF/micron*Le, and
making Ccbx=0
Ree
Rcs
Ree
RL
RL
RL
RL
Please be very careful in the boundary
conditions of the simulation to ensure that
Problem 1: The circuit to the left is one
stage in a string of identical differential
stages. The positive supply is zero volts
and the negative supply is 4.2 volts. Use
the device model above, but scale the
device areaand hence the model element
values so that all the DC bias voltage
drops across RL are 150 mA and the
transistors are biased at 1 mA/micron
current density.
Note critically in this
analysis that the load of each stage is
RL/2 because of the input impedance of
the input impedance of the cascaded
stage.
(this will determine Rcs, Ree).
note that because we are dealing with a
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 Fall '09
 RODWELL
 Alternating Current, Amplifier, Electrical impedance, Voltage drop, emitter degeneration

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